In electrical transformers, the term bushing refers to the hollow electrical insulator that allows electrical conductor cable to pass safely through a conducting barrier without making electrical contact with the transformer or ground. The role of busing is to prevent the occurrence of flashover from the connection of high voltage to an earthing point.
The figure below shows three bushings that are installed on a three-phase small distribution transformer.
Bushings are generally made of porcelain. Sometimes polymers are also used for making bushings.